By the term surge arrester is meant those electrical/electronic devices which, interposed between the active conductors of the electric system and the ground, provide for the discharging to the ground of the overcurrent/overvoltage peaks—e.g., those generated by atmospheric lightning strikes and switching operations—that might otherwise produce serious damage to the electrical system and its apparatuses.
Indeed, the direct lightning phenomena are the main source of devastating destructive effects on electrical systems; indirect discharges and switching surges are also sources of many damages, the origin of which is not easy to identify, but which effects are equally devastating for the sensitive plants and where the operation continuity is essential. The duration of these phenomena varies from a few microseconds up to a few hundred milliseconds, but in this very short time they convey a very high energy content. These phenomena must be properly intercepted in order to protect the plants connected to the main and thus to ensure the integrity and function thereof.
In this context, reference is made to surge arresters of the most recent prior art, comprising a security element in the form of a varistor, which has an equivalent behaviour to that of a variable (non-linear) resistance in term of voltage/current ratio. In the event of an overshot reference voltage, for example when there is a short-term overvoltage/overcurrent peak, the varistor abruptly lowers its resistance, so that the peak can be easily discharged through it, towards the ground, and does not propagate to other parts of the plant with higher resistance. To the electrodes of the varistor the contacts of the connection terminals of the surge arrester are joined electrically, which are in turn connected respectively to a phase conductor and to the protective conductor and/or the neutral conductor. In the internal circuit of the arrester, disposed in series with the protection element as a varistor, a “disconnector” is typically provided, which is a complex disconnecting device known per se, having protective functions in case of failure and/or degradation of the protection element.
The thermal disconnector is substantially constituted by an electric conductor of various shape connected in series with the electrode of the varistor. It consists of a complex unit, typically comprising an elastic metal plate attached to the electrode of the varistor by welding with a low melting solder dot, which is a material capable of melting at relatively low temperatures (120-180° C.). The elastic plate is welded in an elastically flexed or spring-loaded condition, however placed in a resiliently loaded condition such as to define a bias, which tends to distance it from the electrode of the varistor. Thanks to this arrangement when, as a result of degradation, the varistor starts to discharge to the ground a significant current, which is not transient but continuous in nature, this tends to heat up by Joule effect. This temperature is transferred to the solder dot, and when the temperature of the low melting alloy is reached, the holding capacity of the solder dot is impaired, so as to free the metal plate from the contact with the electrode of the varistor, thus opening the electrical circuit and restoring the safety conditions.
Within certain range of short-circuit current, typically a few tens of amperes, the disconnection system within the arrester is therefore able to perform this disconnection in an autonomous way, i.e. without using other internal or external devices placed in series with the arrester itself.
However, when the internal impedance of the arrester suddenly reaches values close to zero and as a result a short-circuit is generated, it occurs a high-intensity current, which gives rise to an unacceptable condition within the electrical system.
Consequently, a disconnecting device must intervene in order to eliminate this condition. Note, however, that the disconnection obtained with a standard disconnector is not always sufficient. In fact, it should be considered that at the opening of an electric circuit where current is flowing, an electric arc could be created, which seeks to maintain the continuity of the circuit itself. If the arc does not extinguishes by itself or the disconnector is not able to stop it, it creates a dangerous situation both in the arrester (overheating and possible fire and/or explosion) and in the relevant electric plant.
Typically, in the past, devices capable of interrupting significant short-circuit currents, of the order of kArms, were constituted by a overcurrent protection, for example a fuse or a circuit breaker, placed in series with the arrester itself.
More recently, it has been provided a very effective solution, described in EP2790192 in the name of the same Applicant, in which one device includes the disconnection capacity to face slow degradation of the varistor, but also circuit opening means with relative self-extinguishing capability to cope with important short-circuit currents.
This system turned out satisfactory, but the Applicant has noted that there is room for improvement of performance.
In brief, the arrester described in EP2790192 comprises a disconnector, consisting of a flexible metal plate made of conductive material with a geometry such that, in normal operating conditions, maintains an interception slider constrained thereon; the latter has the shape of a slider or mobile carriage with a suitable geometry to intercept and stop the electric arc that would be present during the short-circuit; in a suitable longitudinal recess of the slider a preloaded spring is inserted, suitable to provide the pushing energy to the slider during its operation, which is maintained in compression by the presence of the disconnector itself, which acts as a constraint means.
When high short-circuit currents happen, interruption of the circuit takes place by the fact that the metal plate of the disconnector sublimates, so as to free the slider that in turn intercepts and stops the possible formed electric arc.
However, it was found that the sublimation of the conductive plate generates two effects: on the one hand, the desired effect of elimination of the constrain means holding the slider in its normal operating position, so that the slider is free to move due to the transformation of the potential elastic energy of the spring into kinetic energy; but, on the other hand, the non-desired effect of formation of a conductive gaseous mass, called plasma, which, along with the mains voltage, results in the triggering and the diffusion of the electric arc within the entire arc chamber, i.e., the cavity between the solder dot of the disconnector and the residual root portion of the metallic plate.
In summary, the development of the plasma in the arc chamber causes an instantaneous rise in temperature and pressure.
At the same time, the release of the slider triggers the process that leads to the extinction of the electric arc (well described in EP2790192), but such operation must take place in a sufficiently fast manner so as to prevent the pressure and temperature from being excessively high within the device, up to create explosive effects.
It was found that as the short-circuit current increases, the only potential elastic energy of the spring may be insufficient to impart a thrust to the slider such as to reduce the actuation time and then extinguish the electrical arc in a time span compatible with the mechanical strength of the arrester housing.
In particular, it was noted that the high pressure of the plasma generated by the electric arc exerts on the front end surface of the slider a longitudinal counterthrust, with a direction opposite that produced by the spring, which opposes the movement of the slider. So long as this pressure produces this counterthrust, the slider, although urged by the spring, is not able to move in a manner rapid enough to extinguish the arc within a time span compatible with the mechanical strength of the device housing. The criticality of the phenomenon is inherent in the fact that the counterthrust generated by the plasma pressure increases with the square of the short-circuit current; vice versa, the thrust exerted by the spring is an invariant with respect to this current.
This phenomenon is not mitigated adequately even by the provision of pressure evacuation holes pierced in the slider guide chamber on the back side of the slider itself.
Other arrangements of surge arresters are disclosed also in US20110170217, WO2007/093572, DE102006042028, US20120050935 and EP2725588, but none of them is supplying any useful suggestion to address the above cited technical problems.